The invention concerns the machining of workpieces by means of material-removing, preferably mechanically material-removing, methods and apparatuses in that respect, wherein the workpieces include rotationally symmetrical surfaces which are arranged both concentrically and also eccentrically with respect to the central axis of the workpiece, and possibly end faces extending beyond same, which are to be machined.
A typical workpiece of that kind is crankshafts in which the peripheral surfaces of the main bearings represent the concentric rotationally symmetrical surfaces and the peripheral surfaces of the big-end bearings represent the eccentric rotationally symmetrical surfaces. In addition machining operations on the end journals or end flanges (of small or large outside diameter respectively) which are admittedly concentric but which represent the end region and thus the region for gripping the workpiece in chucks represent a difficulty, and similarly for machining side cheek side surfaces, which involves the removal of large amounts of material.
Crankshafts are typical representatives of workpieces which combine the following problems:                rotationally symmetrical workpiece surfaces which are positioned both concentrically and also eccentrically have to be machined,        in addition end faces have to be machined,        also the end regions of the workpiece, at which the workpiece is normally clamped in the chucks of the machine, also have to be machined, and they must be in conformity with the other regions of the workpiece in terms of roundness and central alignment, to a high degree, and        by virtue of its geometry the workpiece exhibits little resistance in relation to in particular radially applied machining forces.        
The known range of material-removing machining methods is available for machining the individual surfaces, beginning with the chip-cutting machining methods whose tools have a geometrically defined cutting edge. Those methods can be divided into the following two groups:                workpiece-based methods, that is to say methods in which the desired cutting speed (relative speed between the surface of the workpiece and the cutting edge of the tool, which operates thereon) is achieved primarily by the rotational speed of the workpiece: longitudinal turning, face turning, broaching, rotational broaching (the broaching cutting edges are arranged on the periphery of a round main tool body which rotates in the machining operation, but more slowly than the workpiece), turning rotational broaching (supplemental to the above-described rotational broaching, the main tool body also carries turning tools, in use of which the rotational broaching tool does not rotate but is displaced linearly in the X-or Z-direction with respect to the workpiece for longitudinal turning or face turning), finishing (grinding with a substantially stationary finishing tool; even finer grain size than grinding tools), and        tool-based methods in which therefore the cutting speed is produced primarily by the movement, in particular rotational movement, of the tool: orthogonal milling (a milling tool which is disposed with its axis of rotation perpendicular to the rotationally symmetrical surface to be machined machines that surface primarily with the end cutting edges on the face of the milling tool), external milling (a disk-shaped milling cutter whose axis of rotation is parallel to the axis of rotation of the workpiece primarily machines with the cutting edges arranged on its outside periphery, the corresponding peripheral surface of the workpiece), and external round grinding (instead of the above-described disk-shaped milling tool, a disk-shaped grinding disk is used in the same positioning with respect to the workpiece).        
In that respect, the last-mentioned representatives in each of the two groups are already methods with a cutting edge which is geometrically not defined.
In addition there are also methods which remove material without a mechanically operable cutting edge, for example electro-erosion methods, material removal by means of laser and so forth, in which however only slight relative speeds between the tool and the workpiece are necessary and that relative speed can be afforded selectively by movement of the workpiece and/or movement of the tool.
For large-scale mass production of workpieces of that kind such as for example automobile crankshafts a machining time which is as short as possible—including set-up and dead times—for each crankshaft on the one hand and low tool and energy costs on the other hand are the crucial parameters, in dependence on the levels of surface quality (roundness, roughness depth and so forth) which can be achieved in that respect and which can govern the necessity for subsequent final machining steps such as grinding and/or finishing.
In that sense at the present time the machining methods which remove material by means of mechanical cutting are still to be preferred for large-scale mass production.
In that respect at the present time rotational broaching or turning-rotational broaching is in the forefront in regard to concentric rotationally symmetrical surfaces. At the present time external round milling is preferred in regard to the eccentric rotationally symmetrical surfaces, that is to say for example the big-end bearing locations. As the big-end bearing location rotates around the central axis of the workpiece during the machining procedure—so that it is possible to machine all peripheral points from one side—tracking of the corresponding tool, which is highly accurate in respect of time and geometry, is necessary at the same time. In order to be able to implement that, tool-based methods are preferred for machining those eccentric rotationally symmetrical surfaces. When using workpiece-based methods—in order to achieve a high cutting speed and thus efficient machining—the workpiece would rotate so fast that tracking adjustment of the tool would not be a viable option or the rotary speeds of the workpiece, which can be achieved in that way, and thus the cutting speeds, would not be competitive.
The methods which are preferred at the present time are generally used in succession on separate machines in large-scale mass production. In addition—mostly also on a separate machine or station in a production line—the end regions, in the case of a crankshaft therefore the end journals and the end flange, are firstly pre-machined separately at least at the periphery, optionally also at the end face, in order to afford defined clamping surfaces for the further machining procedure.
In accordance with the present application, in regard to the peripheral surfaces to be machined, reference is admittedly made only to rotationally symmetrical surfaces as that is by far the greatest proportion of machining situations involved. It will be appreciated that external round surfaces which are not rotationally symmetrical but convexly curved, such as for example the cams of camshafts, can also be similarly machined.
Occasionally consideration has also been given, for dealing with small numbers of items such as a pre-production design of crankshafts and so forth, for the machining of the concentric rotationally symmetrical surfaces to be effected by workpiece-based machining methods and for the machining of the eccentric rotationally symmetrical surfaces to be effected by tool-based machining methods on one machine, insofar as the two appropriate tool units are both present there. In that respect the extremely different rotary speed ranges to be implemented for the workpiece drive represented the one major problem and machining of the end regions of the crankshaft represented the other major problem.